Previously, a common technology for extracting pure hydrogen from industrial streams, such as for hydrogenation for changing the balance of hydrogen in those streams, or to increase reaction selectivity was to use membranes of palladium or palladium alloys often supported structurally by a porous ceramic matrix. A problem exists with this technology because palladium and its alloys are not strong and are very expensive. Accordingly, membranes which contain thick enough palladium layers to be made without holes and not break tend to be expensive and have relatively high resistance to hydrogen permeation. By contrast, refractory metals and alloys, especially those based on (Nb, V, Ta, Ti, Zr) have greater strength than Pd and Pd based alloys, they are also cheaper per unit volume and most have greater intrinsic permeabilities to hydrogen, but their surface properties are not good.
For example, the U.S. Pat. No. 1,292,025 to Darling discloses a membrane requiring porous or discontinuous palladium. The U.S. Pat. No. 4,496,373 to Bohr et al discloses alloying the palladium layer with silver, calcium, or yttrium. The patent also requires an intermediate melt layer. The U.S. Pat. No. 4,536,196 to Harris discloses essentially a palladium membrane which is coated with various metals as poisons to prevent the fouling of the palladium surface. Under some circumstances, this poisoning can be advantageous to the surface properties of the membrane, but the high cost and low reliability of palladium remains. The U.S. Pat. No. 4,313,013 to Harris shows similar palladium membranes that have been in use.
The U.S. Pat. No. 3,350,846 to Makrides et al discloses a process of purification of hydrogen by diffusion through a very thin membrane of palladium coated Group V-B metal. Aside from ignoring the more favorable metal alloys, the patent incorrectly identifies the source of permeation resistance intrinsic to the membrane, and thus derives the necessity of using membrane thicknesses from about 0.001 inch to 0.020 inch. The presumption is that these thicknesses are required to get reasonable rates of permeation. In part, this application is that this choice was based on an erroneous presumption that they were measuring the intrinsic permeability of the membrane and not a local, interface resistance localized at the Pd refractory interface. The deleterious interface resistance is a product of their method of membrane manufacture. The low limit on membrane thickness which they propose seems insufficient to withstand the most desirable applied pressures without rupture, and may preclude production of membrane without pin holes and voids in the membrane. Small diameter tubes or extra strength alloys would be a logical answer here but since this patent limits itself to pure metals which are sputter-coated, small diameter tubes coated on the inside and out would be very difficult to produce. The patent discloses membranes using Group V-B metals having a palladium layer on each side thereof. The palladium in this patent is applied to the Group V-B metal after an initial surface treatment consisting of the steps of etching electrolytically in hydrofluoric acid and washing with acetone. While the foil is still wet with acetone, it is placed in a vacuum chamber where it is dried by evacuation, after which palladium is sputtered onto both sides to form a very thin film which is free of voids, holes and pores. Presumably, these thin flat foil membranes would either have to be supported in a non-disclosed manner, or be used exclusively with a very low pressure difference between the source gas mix and the recovered H.sub.2. A paper by Hsu and Buxbaum, J. Electrochem. Soc., 132:2419-2420 (1985) presents a coated Zr membrane for use in hydrogen extraction. Because of the choice of refractory materials (i.e. Zr), the membrane is only applicable for applications at extremely low H.sub.2 pressures, the only practical example being the extraction of hydrogen isotopes from a liquid lithium stream in a nuclear reactor environment.
The present invention provides membranes which combine the best properties of palladium and of refractory metals; that is, good strength, good durability, low cost, and low resistance to hydrogen. Selectivity is essentially 100% hydrogen extracted.